Microemulsion process for producing acrylamide-alkyl acrylamide copolymers

ABSTRACT

The copolymerization of water soluble monomers with water insoluble monomers can be effected by dissolving the water insoluble monomer into a homogeneous mixture of oil, surfactant, and cosurfactant and then diluting this with water containing the water soluble monomer to yield a predominently aqueous, homogeneous, transparent dispersion of microemulsion droplets. The polymerization is initiated with water soluble free radical initiators to give copolymers that are free of visible particulates and such that the reaction product remains homogenous. The components are chosen to yield copolymers that are water soluble and which viscosify at low concentrations.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in-part of Ser. No. 454,238, filedDec. 29, 1982, now abandoned.

BACKGROUND OF THE INVENTION

Polyacrylamide and hydrolyzed polyacrylamide are water soluble polymersthat have been previously disclosed in the literature and have foundapplication in the viscosification of aqueous solutions. This isachieved through a combination of high molecular weight and chainexpansion due to repulsion of pendant ionic groups along the polymerchain. However, high molecular weight polymers present well knowndifficulties in manufacture and subsequent processing because of theirirreversible degradation when exposed to conditions of high shear suchas would be obtained in the usual stirring devices. Moreover, thepresence of pendant ionic groups leads to solution properties which aremarkedly influenced by the presence of dissolved cations. In particular,the viscosity of solutions of these polymers usually decrease stronglyupon increasing concentrations of brine.

We have discovered an alternative means for providing polymers whichviscosify water or brine at low concentrations. This method relies onthe incorporation of a small amount of hydrophobic groups into a polymerwith a water soluble backbone. Those hydrophobic groups tend toassociate with one another in an aqueous solution, and when theassociation occurs intermolecularly, the solution viscosity may beincreased relative to the polymer without the hydrophobic side groups.An additional benefit is that the solution viscosity is relativelyinsensitive to salts because the hydrophobic groups are not ionic.

The synthesis of copolymers composed of water soluble and waterinsoluble monomers presents difficulties. In order for polymerization tobe effected, the monomers must obviously come into close proximity toone another. A variety of processes based upon prior art couldconceivably achieve this, but have serious deficiencies, necessitatingthe instant invention. For example, simply dispersing the waterinsoluble monomer as fine particles in the aqueous medium, whiledissolving the water soluble monomer in water would result in poorincorporation of the water insoluble monomer and would lead to aheterogeneous product of particles dispersed in the predominently watersoluble polymer. This would therefore require the extra step ofseparating the unreacted monomer particulates from the reaction product.

Alternatively, both monomers may be dissolved in a solvent or solventmixture having properties intermediate between water and a hydrocarbonsolvent. Although this undoubtably allows the comonomers to come intoclose proximity to one another, since the dispersion is on a molecularscale, this process presents other difficulties. For example, often thecopolymer is insoluble in the mixed solvent which is capable ofsolubilizing the monomers. This leads to precipitation of the copolymerwhen it has a molecular weight which is still too low to produceefficient viscosification. The reaction product is usually heterogeneouswhich therefore requires a disadvantageous additional processing step.Further, the water miscible solvents such as alcohols, acetone, etherand acetic acid are fairly good chain transfer agents and when used inreasonable quantities would lead to decreased molecular weights andhence poor viscosification efficiency.

Conventional emulsion polymerization, which uses a surfactant todisperse the water insoluble monomer into the aqueous medium containingthe dissolved water soluble monomer, has other disadvantages. In thisprocess, the bulk of the water insoluble monomer is contained initiallyin droplets which are at least one micron in diameter. These dropletsmust be stabilized against coalescence by a combination of agitation andadded surfactant. The product copolymer is usually in the form ofparticulates with diameters on the order of 500 to 2000 Å.

SUMMARY OF THE INVENTION

It has been found that a process for producing water soluble copolymersof acrylamide and alkyl acrylamide which are efficient viscosifiers maybe produced by a process wherein the water insoluble alkyl acrylamide isdissolved into microemulsion droplets which are dispersed in watercontaining the water soluble acrylamide and a suitable water solubleinitiator. The microemulsion droplets are sufficiently small, less thanabout 1000 Å in diameter, that they form spontaneously upon mixing thecomponents, are stable toward phase separation, and effectively dispersethe water insoluble monomer on a very fine scale so that thecopolymerization is effected without the formation of latexes or fineparticulates of water insoluble polymer.

The microemulsion is composed of an oil or hydrocarbon which is capableof dissolving the water insoluble alkylacrylamide, a surfactant andcosurfactant which act together to enable very small droplets of thesecomponents to form which can be dispersed into water containing thewater soluble monomer. This microemulsion copolymerization process doesnot result in any phase separation of the polymer or microemulsioncomponents as the reaction proceeds to completion, but remains clear andhomogeneous. The copolymers isolated from the reaction mixture areefficient viscosifiers of water or brine, having molecular weights suchthat their intrinsic viscosities are greater than about 1 dl/g, but notso high that they are extremely susceptible to shear degradation.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a plot of reduced viscosity versus concentration foracrylamide/dodecyl acrylamide copolymers polymerized in a Tween 60microemulsion.

FIG. 2 illustrates a plot of reduced viscosity versus concentration ofacrylamide/dodecyl acrylamide copolymers (polymerized in a Tween-Spanmicroemulsion) and the effect of salt thereupon.

FIG. 3 illustrates a plot of reduced viscosity versus concentration foracrylamide/octyl acrylamide.

FIG. 4 illustrates a plot of reduced viscosity versus concentration ofacrylamide/octyl acrylamide copolymers and the effect of salt thereupon.

DETAILED DESCRIPTION OF THE INVENTION

The process of the present invention overcomes the difficultiesexperienced in conventional polymerization just described. Inparticular, it enables the copolymerization of the water solublemonomer, acrylamide, and water insoluble monomers such as a higher alkylacrylamide. The process relies on cosolubilizing the water insolublemonomer into a predominantly aqueous media by the use of a specialmixture of surfactant, cosurfactant, and hydrocarbon. When mixed with anaqueous solution of the water soluble monomer, these special mixturescan disperse on a molecular scale the water insoluble monomer to formisotropic, clear, homogeneous systems, called microemulsions. Thesemicroemulsion reaction mixtures are free of visible oil droplets orparticulates of the water insoluble monomer. The polymerization cantherefore be initiated by water soluble initiators to give,surprisingly, copolymers of the water soluble monomer and waterinsoluble monomer which are substantially free of visible particulates.The resultant reaction product remains homogeneous. The components ofthe microemulsion can be chosen so as to give copolymers which are watersoluble and which viscosify at low concentrations.

The microemulsion droplets used in this process form spontaneously uponmixing the components together, i.e., they do not require the vigorousmixing conditions that are required for the preparation ofmacroemulsions such as that used in conventional emulsion polymerizationprocesses. The process is further differentiated from conventionalemulsion polymerization processes in that the diameters of themicroemulsion droplets is of the order of 100 to 1000 Å in contrast tothe diameters of the emulsion droplets of at least 10,000 Å. Thus, themicroemulsion reaction mixture is much more stable against demixing thanthe formulations used in emulsion polymerization. Indeed, no stirring isrequired during the course of the microemulsion copolymerization--thedroplets remain extremely finely dispersed throughout. Moreover, theextremely dispersed nature of the microemulsion droplet containing thewater insoluble monomer permits the copolymerization to occur in such away that a water soluble copolymer is produced which does not containparticulates or latices of water insoluble polymer. These would bedetrimental in such applications as secondary oil recovery, whichrequires a product substantially free of pore-plugging particulates.

The microemulsion formulations which may be used in the process aregenerally complex mixtures of a surfactant, cosurfactant, hydrocarbon oroil and water or brine. The water or brine is generally the predominantcomponent, comprising from about 99 to 80% of the total mixture. Thesurfactant used may be any of the water soluble surfactants such assalts of alkyl sulfates, sulfonates, carboxylates and the like, ornonionic such as ethylene oxide-propylene oxide copolymers, orpolyoxyethylene alkyl ethers, etc., or cationic surfactants such asprimary alkylamines, dialkyl secondary amines, or ethoxylated fattyamines. The cosurfactant used in formulating the microemulsion is oftenan aliphatic alcohol containing about four to about eight carbon atoms,but may also be any of the surfactants chosen above. The oil used is onethat is capable of dissolving the water insoluble monomer. Thecosurfactant is then chosen by determining whether a microemulsion canbe formed by, e.g., titrating with it the other components which aredispersed into water at the desired concentrations. This dispersion isusually a macroemulsion. If the cosurfactant is capable of making thisdispersion transparent (indicating droplet sizes less than about 1000 Å)during the titration then it is a candidate microemulsion reactionmixture. Other methods of choosing the cosurfactants and the relativeconcentrations of components may also be used. Compositions within therange of 1:2 to 2:1 cosurfactant/surfactant (on a weight basis) and withan oil/(surfactant+cosurfactant) ratio of 4:1 to 1:4 are useful butnon-limiting guidelines. The composition limits within which thecomposition is clear (i.e. the microemulsion droplets have diametersless than about 1000 Å) are often narrow.

Some further guidance on the formulation of microemulsions which can beuseful may be found in "Microemulsions-Theory and Practice", L. M.Prince, ed., Academic Press, N.Y., N.Y., (1977) p. 33 ff.

Suitable free radical initiators for the microemulsion copolymerizationprocess are peroxides such as hydrogen peroxide, potassium persulfate,alkyl peroxides and the like. The concentration of the free radicalinitiator is about 0.01 to about 0.50 grams per hundred grams ofacrylamide and alkyl acrylamide monomer. The polymerization is conductedin the absence of oxygen at a temperature of about 20° to about 100° C.The polymer may be recovered from the microemulsion reaction mixture byprecipitation by nonsolvents such as acetone.

The water soluble copolymers which are produced by the microemulsioncopolymerization process of the instant invention are characterized bythe formula ##STR1## wherein R₁ is preferably a C₆ to C₂₂ straightchained or branched alkyl or cycloalkyl group, more preferably C₆ to C₂₀and most preferably C₆ to C₁₈, and R₂ is the same or different alkylgroup as R₁, or hydrogen. Typical, but nonlimiting examples of preferredalkyl groups are hexyl, octyl, decyl, dodecyl and hexadecyl groups; X ispreferably about 90.0 to about 99.9 mole %, more preferably about 95.0to about 99.8 mole %, and most preferably about 97.0 to about 99.5 mole% and y is 1-X. These water soluble copolymers are of a sufficientlyhigh molecular weight that they are efficient viscosifiers of water orbrine, but not so high that they are readily susceptible to irreversibleshear degradation. That is, their intrinsic viscosity is greater thanabout 1 dl/g and less than about 10 dl/g.

DESCRIPTION OF THE PREFERRED EMBODIMENT

The following examples explain the invention, which are by way ofillustration, but not of limitation.

In the examples and comparative examples, the alkyl acrylamides usedwere prepared as follows. A 500 ml 4 necked round bottom flask wasequipped with a condenser, thermometer, N₂ inlet, magnetic stir bar anddropping funnel. After purging with N₂, the n-octylamine, 14.35 g (0.11mole) and triethylamine, 12.35 g (0.41 mole) were mixed with 50 ml oftoluene and added to the flask. The acryloyl chloride, 10.0 g (0.11mole) was dissolved in 50 ml of toluene and added to the droppingfunnel. The reaction is exothermic so the temperature was controlled viaan ice bath and the acryloyl chloride was added dropwise so that thecontents of the flask stayed below 40° C. The resulting slurry wasstirred for an additional hour and then filtered to remove thetriethylamine hydrochloride. The filtrate was stripped in a rotaryevaporator to remove toluene. The resulting product was taken up into240 ml of acetone and then cooled to -70° C. in a dry ice bath. Themonomer crystals which resulted were filtered on a coarse filter underN₂ and then vacuum dried at room temperature for one day. A yield of14.3 g (70%) of white crystals was obtained. A melting range of 36 to37° C. was observed. The same procedure was used to prepareN-(n-dodecylacrylamide except 20.36 g (0.11 mole) of n-dodecylamine wassubstituted for the n-octyl amine.

EXAMPLE 1 Acrylamide/Dodecyl Acrylamide//99/1 Mole Percent in Tween-60Microemulsion

In this copolymerization, a nonionic surfactant, Tween-60, supplied byICI Americas, was chosen. Tween-60 is a polyoxyethylene-20-sorbitanmonostearate having the formula ##STR2## wherein u+v+y+z=20.

A microemulsion concentrate was prepared by mixing 15.85 g of theTween-60 surfactant, 8.15 g of the cosurfactant, n-pentanol and 1.0 g ofhexadecane. A clear homogeneous mixture resulted. Then 0.5 g of dodecylacrylamide was dissolved into this mixture.

This was placed in the reactor and then 460 ml of deaerated water wasadded under N₂ and 14.69 g of acrylamide was added. After mixing, themicroemulsion reaction mixture was water-clear and had a low viscosity.The clarity was comparable to the Tween-60 microemulsion itself withoutadded comonomers. The initiator, consisting of potassium persulfate(0.01 g) was added when the temperature reached 50° C. The temperaturewas maintained for 24 hours. The resulting mixture became slightly hazyand slightly foamy, but no phase separation of the components occurred.The polymer was recovered by precipitation into acetone and it wasredissolved into water.

COMPARATIVE EXAMPLE 1 Preparation of Acrylamide/Dodecyl Acrylamide//99/1Mole % in Water

In this polymerization, 460 g of deaerated water was placed into anitrogen purged polymerization reaction vessel and 14.69 g of acrylamideand 0.5 g of solid dodecyl acrylamide were added. Nitrogen wascontinuously bubbled through the mixture, and it was heated to 50° C.The dodecyl acrylamide became somewhat better dispersed at thistemperature. When the temperature reached 50° C., 0.01 g of potassiumpersulfate was added as initiator and the reaction was left at thistemperature for 24 hours. A very viscous mixture resulted but there weresmall particles dispersed throughout, giving it a very cloudyappearance. The polymer was recovered by precipitation into acetone,dried, and redissolved into water at a concentration of 0.5 wt.%.

Relative amounts of particulates in this solution and one prepared fromExample 1 were assessed by determining the volumes of solution thatcould pass through a Nucleopore™ polycarbonate filter which was 13 mm indiameter and had a pore size of 5 m. The copolymer polymerized in water,plugged the filter after only 6 cm³ of solution passed through it. Incontrast, 28 cm³ of solution of the microemulsion polymerized copolymerof Example 1 was able to pass before plugging. Thus, the microemulsioncopolymerization technique provides a product that produces a polymersolution more free of particulates.

One method for ascertaining the incorporation of hydrophobic groups in awater soluble polymer structure involves solubilization of a hydrophobicmaterial which is normally insoluble in the aqueous phase. Thesolubilization properties of the two copolymers of Example 1 andComparative Example 1 were compared by measuring the saturation uptakeof an oil soluble dye into a 0.5 wt.% solution of the copolymer. Theabsorption in a one cm cell at 485 nm was 0.424±0.007 for themicroemulsion polymerized copolymer and 0.080±0.005 for the copolymermade in the absence of microemulsion. This latter value isindistinguishable from the absorption of water saturated with dye. Thisis evidence that the water polymerized product did not incorporate thehydrophobic dodecyl acrylamide, while the microemulsion polymerizedproducts of Example 1 did incorporate dodecyl acrylamide.

EXAMPLE 2 Acrylamide/Dodecyl Acrylamide//98/2 in Tween-60 Microemulsion

This polymerization was carried out in the same manner as Example 4,except that 1.0 g (2 mole %) of dodecyl acrylamide was used to make asomewhat more hydrophobic polymer. The reaction product was similar inappearance to Example 1 and had a similar low microgel content. Thereaction product did not phase separate and was macroscopicallyhomogeneous. However, when the polymer was separated from the surfactantby repeated acetone precipitation, the product was not completelysoluble in water. Approximately 40% of a 1% solution in water containedin a test-tube formed a swollen, turbid gel in the bottom of the tube. Asample of the same polymer added to the Tween-60 microemulsion at thesame concentration, however, formed a uniform, slightly turbid phase.

EXAMPLE 3 Acrylamide/Dodecyl Acrylamide Copolymers

Copolymers of acrylamide and dodecyl acrylamide were prepared, accordingto the procedure of Example 1, with different mole fractions of dodecylacrylamide, y, that were initially contained in the polymerizationvessel. These copolymers were isolated, purified and redissolved inwater to give solutions with known concentrations.

The viscosities of these solutions were measured by means of aContraves™ low shear viscometer model LS30 using a No. 1 cup and No. 1bob. Temperatures were controlled to ±0.1° C., and measurements weremade at a rotational speed that gave a shear rate of 1.28 s⁻¹. Theconcentration dependence of the solutions are plotted in FIG. 1 fordifferent mole fractions of dodecyl acrylamide. As the mole fraction ofhydrophobic dodecyl acrylamide is increased in the copolymers, thespecific viscosity per unit mass of polymer also increases. An importantfeature demonstrated by these data is that the intercepts at zeroconcentration are independent of the mole fraction of dodecylacrylamide. This intercept is the intrinsic viscosity, defined as##EQU1## where η_(o) is the viscosity of the solvent, η is the measuredviscosity of the solution with a concentration of polymer c. Thisintrinsic viscosity is related to the molecular parameters for ahomopolymer by ##EQU2## where N_(A) is the Avagadro number, M_(w) is thepolymer molecular weight and V_(h) is the hydrodynamic volume. Forcopolymers with a repeating backbone unit, the hydrodynamic volume isgiven by: ##EQU3## where R_(G) is the radius of gyration of the polymer,and ξ is a dimensionless constant with a theoretical value of 0.875 forpolymer having a random chain conformation.

The radius of gyration is given by ##EQU4## where β is an effective bondlength and ν is the degree of polymerization. The molecular weight of acopolymer with a mole fraction 1-y of acrylamide (M_(w) =71) and dodecylacrylamide (M_(w) =239) is given by ##EQU5##

This formula permits one to compare intrinsic viscosities foracrylamide/dodecyl acrylamide copolymers in a simple way. For example,the copolymer with x=0.98 can be compared with the homopolymer, whichhas x=1.0: ##EQU6## Thus, if the degree of polymerization for the twopolymers are the same, their intrinsic viscosities should differ by onlyone percent. Conversely, if the intrinsic viscosities of the twopolymers differ by only a few percent, then we may infer that theirdegrees of polymerization are nearly the same. FIG. 1 therefore showsthat the degree of polymerization of the copolymers and homopolymer arevery nearly all equal, and that introduction of the dodecyl acrylamideproduces a beneficial enhancement in viscosity.

Values for the reduced viscosities of some of these systems are shown inTable I at a polymer concentration of one percent, in water and also ina two percent sodium chloride solution. These data show that theviscosities are not lowered by the salt.

                  TABLE I                                                         ______________________________________                                        Alkyl Acrylamide-Acrylamide Copolymer                                         Compositions and Reduced Viscosities                                                             η.sub.SP /C (Water)                                                                    η.sub.SP /C (2% Salt)                     RAM    AM (Mole %) dl/g.sup.(a) dl/g.sup.(a)                                  ______________________________________                                        --     100         5.2          --                                            C.sub.12                                                                             99.5        7.8          8.4                                           C.sub.12                                                                             99.0        18.4         16.5                                          C.sub.12                                                                             98.0        32.1         34.7                                          C.sub.12                                                                             80.0        .sup. --.sup.(b)                                                                           .sup. --.sup.(b)                              C.sub.8                                                                              99.5        8.3          8.5                                           C.sub.8                                                                              99          18.3         19.7                                          C.sub.8                                                                              98          308          306                                           ______________________________________                                         .sup.(a) At 1% polymer concentration                                          .sup.(b) Insoluble                                                       

EXAMPLE 4 Acrylamide/Dodecyl Acrylamide//99.5/0.5 in Tween 20/SPAN 20Microemulsion

A copolymer of 99.5 mole % acrylamide and 0.5 mole % dodecylacrylamidewas prepared in a manner similar to that described in Example 1 exceptthat SPAN 20 surfactant was used as a cosurfactant. A microemulsionconcentrate consisting of 10 parts by weight of Tween 20 surfactant, 1part by weight of SPAN 20 surfactant and 1.2 parts by weight ofhexadecane was prepared by mixing the components at a temperature ofabout 80° C. and subsequently cooling it to room temperature to give ahomogeneous fluid. No alcohol was used. To 25 g of this mixture, 0.25 gof dodecylacrylamide was added, then after dissolution of it, 14.76 g ofacrylamide and 475 ml of water was added. This mixture was purged withnitrogen. The temperature was brought to 70° C. to give a clear, stablemicroemulsion reaction mixture. Then 0.01 g of potassium persulfateinitiator was added. The mixture was stirred under nitrogen for 24hours. The polymer was precipitated with acetone and vacuum dried.

This polymer was dissolved in water and viscosities as a function ofconcentration were measured as previously described. Similarconcentrations of polymers were prepared in a two weight percent sodiumchloride solution and the viscosities were also measured. The resultsare compared in FIG. 2, which shows that the viscosities aresubstantially unchanged by the presence of sodium chloride. Thiscontrasts sharply with the well-known acrylamide/sodium acrylatecopolymers.

EXAMPLE 5 Acrylamide/Octyl Acrylamide Copolymers

Copolymers of acrylamide/octyl acrylamide were prepared according to theprocedure of Example 1. FIG. 3 gives results for their reduced viscosityas a function of concentration in aqueous solutions for the formedcopolymers. These plots clearly demonstrate the improved thickeningefficiency resulting from the introduction of a small mole fraction ofoctyl acrylamide into the polymer. The salt independence of theviscosity is also shown in Table I. Plots with and without 2% NaCl arealso compared in FIG. 4 for the 98/2 acrylamide/octyl acrylamidecopolymer.

What is claimed is:
 1. A free radical microemulsion copolymerizationprocess for the formation of a copolymer of acrylamide and awaterinsoluble alkyl acrylamide corresponding to the formula below whichcomprises the steps of:(a) forming a microemulsion concentrate whichincludes a surfactant, cosurfactant, and oil for hydrocarbon solvent;(b) dissolving the alkyl acrylamide into this concentrate; (c) addingthe water or brine containing the water soluble monomer to saidconcentrate to form a clear homogeneous microemulsion reaction mixturehaving microemulsion droplets of about 100 to 1000 A; (d) deaerating themixture by passage of an inert gas therethrough; (e) adding sufficientquantity of free radical initiator to initiate copolymerization of thewater soluble monomer and the water insoluble monomer; and (f)continuing the copolymerization reaction for a sufficient time at asufficient temperature to form said copolymer without phase separationoccurring or the formation of substantial amounts of particulates, saidcopolymer having an intrinsic viscosity of about 1 to about 10 dl/g saidcopolymer having the formula: ##STR3## wherein R₁ is an alkyl orcycloalkyl group having about 6 to 22 carbon atoms, R₂ is the same ordifferent alkyl group as R₁, or hydrogen, x is about 90.0 to about 99.9mole %, and y is about 10.0 to about 0.1 mole%.
 2. A free radicalmicroemulsion copolymerization process according to claim 1, furtherincluding means for recovering said copolymer from said reactionmixture.
 3. A free radical copolymerization process according to claim 2wherein said R₁ is an alkyl group having about 6 to about 18 carbonatoms.
 4. A free radical copolymerization process according to claim 2wherein x is about 95.0 to about 99.8 mole %, and y is about 5.0 toabout 0.2 mole %.
 5. A free radical copolymerization process accordingto claim 2 wherein said reaction solution is maintained at a temperatureof at least about 0° C. for at least about 0.5 hours.
 6. A free radicalcopolymerization process according to claim 1 wherein said surfactant isa poly-oxyethylene-2-sorbitan monostearate.
 7. A free radicalcopolymerization process according to claim 6 wherein said free radicalinitiator is potassium persulfate.
 8. A free radical copolymerizationprocess according to claim 2 wherein said alkyl acrylamide is N-octylacrylamide.
 9. A free radical copolymerization process according toclaim 2 wherein said alkyl acrylamide is N-dodecyl acrylamide.
 10. Afree radical copolymerization process according to claim 2 wherein saidalkyl acrylamide is N,N-dioctyl acrylamide.